Method of manufacturing magnetic cores



Jan. 24, 1961 G. A. SMITH 2,968,862

METHOD OF MANUFACTURING MAGNETIC CORES Original Filed July 25, 1955 F G. 6. INVENTOR.

GEORGE A. SMITH United States Patent METHOD OF MANUFACTURING MAGNETIC CORES George A. Smith, Pine Blulf, Ark., assignor to Central Transformer Corporation, Pine Bluff, Arln, a corporation of Arkansas Original application July 25, 1955, Ser. No. 524,049. Divided and this application May 14, 1956, Ser. No. 584,665

2 Claims. (Cl. 29-155.7)

This invention relates to certain new and useful improvements in the method of manufacturing magnetic transformer cores and the product obtained therefrom.

This application is a division of my copending application Serial No. 524,049, filed July 25, 1955.

It has been a well-known practice in the art of manufacturing magnetic transformers to wind the coil independently and subsequently either join a pre-formed core with the coil or build up a core around the coil. Since in the final form of the transformer the coil and the core both form closed loops linked with one another, rather than wind one about the legs of the other, it has been expedient to provide a break in the core whereby the core and the coil may be linked. The problem of subsequently joining the ends of the laminations or turns of the core in an efficient manner has heretofore been an ever-present problem in the field o-f transformer manufacturing. Heretofore, the individual lamination layers or turns of transformer cores have been joined by either the so-called butt joint method in which the ends of the lamination layers or turns are abutted or the so-called lap joint in which the ends of the laminations have been joined by lapping one end over the other. In the butt joint method the magnetiting current has been high due to the fact that the core has a high resistance at the joints. In the lap joint method an excessive build-up is created in the area of the joints since the layers are doubled in this area. This excessive build-up is undesirable since it creates a transformer which is unduly bulky. Additionally in this method there is a waste of material since the lapped portion of the joint does not add to the effective length of the core.

The present invention is directed towards providing a method of forming a transformer core and the product obtained therefrom in which the above-mentioned problems encountered with the so-called butt joint and socalled lap joint are alleviated.

It is contemplated that a transformer core fabricated by the method of the present invention has an improved joint whereby a transformer constructed with said core will have the characteristics of low core loss. In such a transformer core the build-up in the joint area will be reduced and there will be a savings in core material over the lap-joint method.

Further it is contemplated that such a transformer core will be provided with a joint in which the ends of the individual lamination layers are lapped and one of said ends abutted with an adjacent layer, whereby a joint is provided having a build-up of one and a half times the thickness of the other legs of the core rather than twice the thickness as in the conventional lap joint.

Further it is contemplated that an alternate embodiment of the subject invention will be provided with a joint in which the ends of successive pairs of lamination layers are lapped, with one of said ends being abutted with an adjacent layer, and with a plurality of shortened lamination layers being respectively interposed between Said successive pairs and abutted with a pair of ends "ice thereof, whereby a joint is provided having the same thickness as the other legs of the core.

It is an object of the present invention to provide new and useful improvements in the method of manufacturing magnetic transformer cores.

It is a further object of the invention to provide such a method which includes the steps of winding magnetic strip material on a trapezoidally-shaped mandrel to form a core winding, annealing said core winding in wound condition, cutting through the longer of the unequal legs of said core winding 01f center from the vertical center line of said longer leg, building a core by alternately reversing lamination layers from said core winding and forming a joint in which the ends of the individual lamination layers will be lapped and one of said ends abutted with an adjacent layer.

A further object of the present invention is to provide such a method of forming a transformer core which includes the additional steps of removing every third lamination layer of said core winding and cutting a segment therefrom, and interposing said third lamination layers between successive paIrs of lappzd lamination layers in said core, abuiting ends of lapped layers, to close the gaps formed by the lapped laminations whereby a core is provided having uniform leg thickness.

A further object of the invention is to generally improve the design and construction of transformer cores and the method of making same.

The means by which the foregoing and other objects of the present invention are accomplished and the manner of their accomp'ishment will be readily understood from the following specification upon reference to the accompanying drawings, in which:

Fig. l is a front elevational view of a core winding of magnetic strip material for a transformer core, as illustr ted on a mandrel upon which it has been wound.

Fig. 2 is a front elevational view of the core winding after annealing and severing.

Fig. 3 is a front elevational view of the completed core.

Fig. 4 is a front elevational view of a core winding of magnetic strip material for a tran former core of an a ternate cons ruction as i.lustrated on a mandrel upon which it has been wound.

Fig. 5 is a front elevational view of the core winding of Fig. 4 after annealing and severing.

Fig. 6 is a front elevational view of a stack of lamination layers removed from the seve ed core winding of Pig. 5, after having a segment cut therefrom.

Fig. 7 is a front elevational view of the completed transformer core of alternate construction.

Referring now to the drawings in which the various parts are indicated by numerals, ferrous strip material having suitable magnetic properties for use in transformer cores is first wound on a trapezoidal mandrel to produce a containuous coil of strip material. Thus as illustrated in Fig. 1, a strip material 11 introduced to the trapezoidal mandrel 13, from a source of supply not shown, is secured to the mandrel in any desired manner, and the mandrel is rotated, in the embodiment illustrated, in a clock-wise direction to produce core winding 15. During the winding operation independent spacers 17 are respectively inserted between successive pairs of lamination layers in the longer leg 19 of the unequal legs 19, 21 of core winding 15. The term lamination layers is used herein to refer to the individual layers since one or more laminations of magnetic strip material may be included in each of said layers.

Spacers 17 are preferably of a thickness equal to the thickness of an individual lamination layer whereby the total build-up of thickness of the leg 19 will be equal to one and one-half times the thickness of the shorter leg 21 of the two unequal legs and similarly one and a half times the thickness of the two equal winding legs 23, 25 of core winding 15. As will be more apparent in the description of the formation of the final core, spacers 17 provide the necessary space factor as indicated at 26 between successive pairs of lamination layers in order to accommodate the free ends of the successive pairs of lamination layers and to insure the proper length thereof.

After the desired number of turns of lamination layers have been wound on the mandrel 13, which is governed by the particular transformer design required, the strip material 11 is severed from the core winding 15, which is then removed from the mandrel and bonded with suitable bonding straps, whereby the shape of the core winding as formed on the mandrel is substantially maintained.

Spacers 1-7 are removed from core winding 15 and the core winding placed in a suitable oven or other heating device wherethe core winding is annealed at the proper temperature to remove the stresses set up during the winding process, thereby improving the magnetic properties of the strip. Additionally the annealing sets the shape of the core winding 15 substantially as bound.

After core winding 15 has been annealed the longer leg 19 of the core, is severed to produce discontinuous lamination layers, as iIlustrated in Fig. 2. The discontinuous lamination layers, beginning at the inner layer and continuing outwardly for convenience, are herein designated as at 30, 31, 32, 33, 34, 35, 36, 37, 38 and 39 respectively.

The severing of the longer leg 19 may be accomplished as by means of a band saw or other cutting means, the out being made transversely through the material of the leg. along a line laterally offset from the vertical center line of leg 19, as for example to the right of the center line of leg 19, as illustrated in Fig. 2. The amount of offset of the cutting line is determined by the desired amount. of overlap of the individual lamination layers of thecompleted core. Also the amount of overlap desired inthecompleted core should be taken into consideration in determining the dimensions of trapezoidal mandrel 13. It will be understood that the amount of overlap of the individual lamination layers necessary to produce a substantially rectangular final core will equal the amount that the longer of the two unequal sides of mandrel 13 exceeds the shorter of the two unequal sides ofthe mandrel. It follows that the longer leg 19 of core winding 15 having been wound on mandrel 13, dimensioned as hereinabove described, will exceed the shorter leg 21 by an amount equal to said overlap of individual lamination layers in the final core. Having selected the desired amount of said overlap of individual lamination layers, which will be hereinafter referred to as overlap length, the severing of leg 19 is preferably made along a line offset from the center line of leg 19 by an amount equal to said overlap length. Referring to Fig. 2, the freev ends of the discontinuous lamination layers thus formed are herein respectively designated with the sufiixes A and B, the longer free ends formed from leg 19 being designated with the sufiix A and the shorter free ends with the sufiix B. Thus the longer free ends of lamination layers 30, 31, 32, 33, 34, etc., are designated 30A, 31A, 32A, 33A, 34A, etc., and the shorter free ends 30B, 31B, 32B, 33B, 343, etc.

The lamination layers of severed core winding 15 are then removed from the core winding and restacked to form completed core 41, as best illustrated in Fig. 3. In assembling the completed core, first the innermost lamination layer 30 may be linked with a pair of coils 43 by spreading the free ends 30A, 30B, inserting said ends through the windows 44 of coils 43 and substantially joining the ends by overlapping free end 30B on free end 30A, said free ends preferably overlapping by an amount equal to said overlap length. It will be understood that free end 3013 terminates substantially at the vertical center line of the upper leg 45 of core 41 and free end 30A terminates at a point offset to the right of said center line by an amount equal to said overlap amount. Next the lamination layer 31 may be similarly linked with core 43 and concentrically stacked over layer 30, but before insertion through the windows of coils 43 layer 31 is reversed, that is, rotated l relative to layer 30. After linking with coils 43 the free ends of layer 31 are joined by overlapping free end 31A on free end 313 and the distal end of free end 31B is abutted with the distal end of free end 30B. it will be understood that by rotating layer 31 with respect to layer 30, free end 31A will terminate at a point which is offset to the left of the vertical center line of leg 45 and by an amount substantially equal to said overlap amount.

Layer 32 is then linked with coils 43 and concentrically stacked over layer 31 with free ends 32A, 32B joined in a manner similar to the joining of the free ends of layer 30. Thus free end 32B is overlapped on free end 32A, which terminates at a point which is offset to the right of the vertical center line of leg 45 by an amount substantially equal to said overlap amount. Next, lamination layer 33 is linked with coils 43 and concentrically stacked over layer 32, but is rotated relative to layer 32 b efore insertion through the windows of coils 43. After linking, the free ends of layer 33 are joined similar to the joining of layer 31. Thus free end 33A is overlapped on free end 3313 and the distal end of free end 33B abutted with the distal end of free end 32B.

In a like-manner the remainder of the core is assembed by successively linking the layers 34, 35, 36, 37, 38 and 39 with coils 43 and joining the free ends. Thus the odd numbered layers 35, 3.7, and 39 are rotated 180 relative to the even numbered layers 34, 36 and 3.8, the free ends 348, 35A, 36B, 37 3313, 39A are re spectively overlapped on free ends 34A, 35B, 36A, 3713, 38A, 39B, and the distal ends of free ends 35B, 37B, 39B respectively abutted with the distal ends of free ends 34B, 36B, 38B whereby a joint is formed in which free ends 35A, 37A and 39A terminate along a line which is offset to the left of the center line of leg 45, free ends 34A, 36A and 38A terminate along a line which is offset to the right of the center line of leg 45 and free ends 34B, 35B, 36B, 37B, 38B and 39B terminate along a line which is substantially coextensive with the center line of leg 45.

It will be understood from the foregoing description that a core 41'formed in accordance with the herein described process will have a build-up or thickness in leg 45, wherein the joint is formed, which will equal one and one-half times the thickness of the opposite leg 47 or the side legs 49, 51. In addition, it is apparent that such a core is formed in which there is a concentrically stacked series of lamination layers, each having their free ends overlapped and the distal end of a free end of one of said pair in abutment with the distal end of a free end of the other of said pair.

It will be apparent that a pair of cores may be formed by winding two core windings as described for the formation of core winding 15, severing said pair of core windings through the longer of their unequal legs, one core being severed off center to one side of the center line by the same amount and combining the lamination layers of the core windings thus severed in a manner similar to the assembly of core 41. Thus the even numbered turns from one of said core windings may be combined with the odd numbered turns from the other of said core windings to form one core and the odd numbered turns from said one of said core windings may be combined with the even numbered turns from said other of said core windings to form another core. It is obvious that the alternate layers do not have to be rotated 180 as in the formation of core 41 since with the use of two core windings respectively severed at oppositely offset points from the'center line the same effect is obtained.

:to said overlap amount.

An alternate method of forming a core in accordance with the present invention includes first winding suitable ferrous strip material on a trapezoidal mandrel to produce a continuous coil of strip material. Thus, as illustrated in Fig. 4, strip material 11 introduced to a trapezoidal mandrel 13 from a source of supply, not shown, is secured to the mandrel in any desired manner, and the mandrel is rotated, in the embodiment illustrated, in a clockwise direction to produce core winding 53. The formation of core winding 53 is similar to the method heretofore described for the formation of core winding 15 except the spacers 17 are omitted in the processing of core winding 53. Similar to the processing of core winding 15, core winding 53 is removed from the mandrel 13, bonded with suitable bonding straps and annealed in an oven.

After annealing, the longer leg 55 of the two unequal legs 55, 57 of core winding 53 is severed to produce discontinuous lamination layers as illustrated in Fig. 5. The discontinuous lamination layers, beginning at the inner layer and continuing outwardly, will be herein designated as at 60, 61, 62, 63, 64, 65, 66, 67, 68 and 69 respectively.

Similarly to the severing of core winding 15, the severing of leg 55 is made transversely through the material of the leg along a line offset from the center line of leg 55, as for example to the right of the center line of leg 55, as illustrated in Fig. 5. The selection of the amount that the cutting line is offset will, just as heretofore described for the method of forming core winding 15, be dependent upon the desired overlap amount in the final core. Similarly the dimensions of the trapezoidal mandrel will be dependent thereon. The free ends of the discontinuous lamination layers formed by severing leg 55 as herein above described will herein be designated by the suffixes A and B, the longer free end being designated with the suffix A and the shorter free end with the suffix B. Thus the longer free ends of lamination layers 60, 61, 62, 63, 64, etc., are designated 68A, 62A, 63A, 64A, etc., and the shorter free ends 6013, 61B, 62B, 63B, 6413, etc.

After severing, every third lamination layer is removed from severed core winding 53 and may be restacked to provide a stack of third laminations 62, 65, 68. A segment of each of the longer free ends 62A, 65A, 68A of layers 62, 65, 68 is then cut off, as by means of a band saw or other cutting means, leaving free ends 62C, 65C and 68C. Preferably the length of said segment is equal to said overlap length whereby free ends 62C, 65C and 68C will be equal to free ends 62B, 65B and 68B, and the gap between the distal ends of said free ends will be equal to twice said overlap length.

The lamination layers 60, 61, 63, 64, 66, 67, 69, remaining after removal of every third lamination layer, and said third lamination layers 62, 65, 68, each having a segment removed as above described, are assembled together to form a final core 71 of generally rectangular shape. In general the method of assembly comprises assembling lamination layers 60, 61, 63, 64, 66, 67, 69 in a manner similar to the formation of core 41 and including additionally the step of respectively interposing lamination layers 62, 65, 68 between successive pairs of lamination layers 60, 61, 63, 64, 67, 69 to fill the gaps formed by the joints thereof whereby a core is formed having legs 73, '75, 77, 7%, of substantially equal thickness.

Thu-s in assembling core 71 first the innermost lamination layer 66 may be linked with a pair of coils 81 and the ends of layer 60 joined by overlapping free end 6613 on free end 60A by an amount equal to said overlap length. It will be understood that free end 693 terminates substantially at the mid-point of the upper leg 73 and core 71 and free end 60A terminates at a point offset to the right of said mid-point by an amount equal Next lamination layer 61 may be similarly linked with coils 81 and concentrically stacked over layer 60, but before insertion through the windows 82 of coils 81 layer 61 is rotated relative to layer 60. After linking with coils 81 the free ends of layer 61 are joined by overlapping free end 61A on free end 61B and abutting the distal end of free end 61B with the distal end of free end 60B. It will be understood that by rotating layer 61 with respect to layer 60 free end 61A will terminate at a point which is offset to the left of the mid-point of leg 45 and by an amount substantially equal to said overlap amount.

Layer 62 is then linked with cois 81 and concentrically stacked over layer 61 with the distal end of free end 62C abutting the distal end of free end 61A.

Next layer 63 is linked with coils 81 and concentrically stacked over layer 62 with the distal end of free end 63A abutting the distal end of free end 623 and free end 638 overlapping free end 63A. Layer 64 is rotated 180 relative to layer 63, then linked with coils 81, and concentrically stacked over layer 63 with the distal end of free end 648 abutting the distal end of free end 6313 and free end 64A overlapping free end 64B.

Layer 65 is then linked with coils 81 and stacked over layer 64 with the distal end of free end 65C abutting the distal end of free end 64A.

In a like manner the remainder of the core is assembled by successively linking the layers 66, 67, 68, and 69 with coils 81 and joining the free ends of the layers. Thus the free ends 66B, 67B. 69B are respectively overlapped on free ends 66A, 67A, 69A, the distal end of free ends 66B, 68C, 68B respectively abutted with the distal end of free ends 67B, 67A, 69A, whereby a joint is formed in which free ends 61A, 64A, 67A respectively abut free ends 62C, 65C, 68C along a line which is offset to the left of the center line of leg 73 by an amount equal to said overlap length, free ends 63A, 66A, 69A respectively abut free ends 62B, 65B, 68B along a line which is offset to the right of said center line by a similar amount, and free ends 60B, 63B, 668 respectively abut free ends 61B, 64B and 678 along a line substantially co-extensive with the center line of leg 73.

It will be apparent that a pair of cores similar to core 71 may be formed from a pair of core windings similar to core windings 53 by severing one core winding offset to the left and the. other offset to the right which will give the same result as reversing or rotating lamination layers from a single core winding.

Thus it is apparent from the foregoing description that a core or cores fabricated in accordance with the alternate method will comprise a concentrically stacked series of lamination layers, each series comprising a pair of lamination layers, each having their ends overlapping and each of said pair having one of their ends in abutment with one of the ends of the other of said pair and shortened lamination layers respectively interposed between said series and the ends of which are in abutment with the ends of said series, providing a core of uniform cross-sectional thickness throughout its length.

I claim:

1. A method of fabricating laminated transformer cores to produce cores having overlapped joints in lamination layers and butt joints between adjacent overlapping laminations layers comprising the steps of winding magnetic strip material on a trapezoidal mandrel to form a trapezoida 1y shaped core winding having a pair of spaced unequal legs joined by a pair of spaced and opposite equal legs in which the length of one of said unequal legs exceeds the length of the other unequal leg in an amount substantially equal to the length of overlap of the respective overlapped joints, annealing said core winding substantially in its wound shape to remove the strains therein, cutting through the longer one of said unequal legs along a line offset to one side of the vertical center line of said longer one of said unequal legs a distance substantially equal to said length of overlap to form a plurality of concentrically-stacked discontinuous lamination layers, each having a long ,free end and a short free end, removing every third lamination layer from said plurality of discontinuous lamination layers, cutting a segment having a length substantially equal to said length of overlap from the long free end of each said third lamination layer, concentrically stacking said discontinuous lamination layers in the same relative position as they were originally stacked and during said stacking reversing a first plurality of alternate said discontinuous lamination layers to form a final core comprising a series of successive pairs of lamination layers respectively having said third :laminations interposed between said pairs, each said pair comprising one of a second plurality of said discontinuous lamination layers concentrically stacked with one of said reversed first plurality of discontinuous lamination layers, concurrently with said stacking forming a joint in one of the legs of said core by overlapping the short free ends of said second plurality of discontinuous lamination layers re- ;spectively with the long free ends thereof offset to one side of the vertical center line of said one core leg, by ;overlapping the long free ends of said first plurality of discontinuous lamination layers with the short free ends thereof offset to the other side of the vertical center line of said one core leg by respectively abutting the short free ends of said first plurality of discontinuous lamination layers with the short free ends of said second plurality of discontinuous lamination layers substantially at the vertical center line of said one-core leg, and by .irespectively abutting one ,end :of said third lamination layers with thelong 'free ends of said first'plurality of a'discontinuous lamination layers and the other end of -saidthird lamination layers with the long free end-s of said second plurality of discontinuous lamination layers.

2. A method of fabricating laminated transformer cores to produce cores having overlapped joints in lamination layers and butt joints between adjacent overlapping lamination layers comprising the steps of winding magnetic strip material on a trapezoidal mandrel to form a core winding having a pair of unequal length legs, bindingsaid core winding to hold the winding in substantially its wound shape, annealing said core ,vvind- .ing in; said shape toremove the strains therein, seyering L said core winding across the longer ofsaid unequallegs at a lineoifset from the vertical center line. ofls aidleg ,intan amount equal to the length of the overlapinthe lamination layers to form a plurality of concentrically stackedvdiscontinuous lamination layers, each havinga pair of free ends, removing every third lamination layer from said plura'ity of discontinuous lamination layers, cutting a segment having a length substantially equal to the length of overlap of the respective overlapped joints from the longer free end of each said third'lamination layer, concentrically stacking said discontinuous lamination layers in the same reative position as they were originally stacked and during said stackingreversing a rst plurality ofalternateqsaid.discontinuous lamination layers to form a final core comprising a series of successive pairs of lamination layers respectively having said every third laminations interposed between said pairs, each said pair comprising one of a second plurality of said discontinuous lamination layers concentrically stacked with one of saidfirst plurality of sa discontinuous lamination layers, forrnin" a joint in said core by respectively overlapping the short free ends of said first plurality with the long free ends thereof and the short free ends of said second plurality with the long free ends thereof, and by respectively abutting the short free ends of said first plurality of discontinuous lamination layers with the short free ends ofsaid second pluralityof discontinuous lamination layers, and abutting the ends of said third lamination layers with the longfreeends of said first plurality and said second plurality of lamination layers.

'References'fited in the'file of this patent UNTTED STATES PATENTS Re. 23,987 'Sefton et a1. Apr..26, 1955 2,595,820 Somerville Mayi6, 1952 2,700,207 Zimsky Jan. 25, 1955 

